What happened
Test-flying a repaired P-51 Mustang at about 10,000 ft, Slayton saw a white object at his altitude. He first took it for a kite, then for a weather balloon about three feet across. When he turned to make a pass at it, it looked like a disk on edge, and he realized he was not closing on it at about 280 mph. It then climbed away at about 45 degrees and disappeared. The Blue Book record describes two rapidly revolving discs overlapping each other. Brad Sparks lists it as case 496 in his catalog of Blue Book unknowns.